1. Field of the Invention
The invention relates to a projection type display device for enlarging and projecting an image displayed on a light bulb onto a screen, and, more particularly, to a projection type display device equipped with an improved integrator for providing uniform distribution of illumination with which a light bulb is lighted up.
2. Description of Related Art
Relay-condenser illumination systems, which have been known as a kind of Kohler's illumination system, form an image of a light source at a focal point of a condenser lens which is in a close proximity of a pupil of a projection lens and project the light source image at an infinite distance. Such a relay-condenser illumination system prevents or significantly reduces uneven illumination due to uneven luminance intensity distribution of the light source. The issue of uneven illumination still remains in the relay-condenser illumination system with a light source having an uneven luminance intensity distribution characteristic such as a xenon lamp, a halogen lamp and a metal halide lamp. There have been known various integrator type illumination systems incorporating lens arrays or lenticular plates which eliminate uneven illumination due to uneven luminance intensity distribution of a light source. Such an integrator type illumination system is known from, for example, Japanese Unexamined Patent Publication No. 3-111806.
Further, various forms for dividing a bundle of rays have been known in the field. The integrator type illumination system includes a first integrator optical plate (which is generally called a second fly-eye lens plate), a second integrator optical plate (which is generally called a first fly-eye lens plate) and a field lens disposed in an optical axis in this order after a concave mirror type illumination optical arrangement. Each of the first and second integrator optical plate consists of a number of lens elements arranged in an ordinary matrix thereon. The first integrator optical plate divides a bundle of rays from the concave mirror type illumination optical arrangement which has uneven luminance intensity into bundles of rays as many as the number of lens elements thereof. The respective bundle of rays are reduced in unevenness of luminance intensity as compared with the original bundle of rays. The second integrator optical plate deflects and directs the respective bundles of rays toward an area to be illuminated. The bundle of rays are overlapped on the area by the field lens to form illumination with less uneven luminance intensity distribution.
As one of the integrator type illumination systems provided with two integrator optical plates, Japanese Unexamined patent publication No. 6-265887 describes a projection type display device incorporating a multiple light source arrangement which has a plurality of light sources disposed symmetrically with respect to an optical axis with intention to provide a satisfactory quantity of illumination light. Typically, such an illumination device forms a secondary light source on a surface of the second integrator plate in which a pupil of the projection lens is situated. The secondary light source has uneven luminance intensity distribution on the surface of the second integrator plate owing to the difference among luminance intensity distribution characteristics of the light source arrangements. In particular, the projection type display device with a multiple light source arrangement which includes a plurality of light sources disposed symmetrically with respect to an optical axis provides luminance intensity distribution on a surface of the second integrator plate, in which a pupil of the projection lens is situated, with a peak at a certain distance from the optical axis. The projection lens typically has an image forming characteristic that shows the best performance up close to the optical axis and is gradually aggravated with an increase in lateral distance from the optical axis. Therefore, in the case where a peak of luminance intensity distribution, which governs an image forming quality, is situated on a plane including the pupil of the projection lens at a certain distance from the optical axis, it is difficult for the projection lens to fulfill its essential image forming performance.
In order to eliminate the problem, the applicant of this application has proposed a projection type display device with a multiple light source arrangement in Japanese Unexamined Patent Publication No. 11-44920. This projection type display device two wedge-shaped prisms disposed at a specified axial separation between a light source arrangement and an integrator or incorporated in the integrator. The first wedge-shaped prism reflects a bundle of rays from the light source arrangement toward the optical axis, and then the second wedge-shaped prism reflects the bundle of rays so as to be parallel to the optical axis. While the projection type display device is superior on the point that it secures the essential image forming performance of the projection lens by means of shifting a peak of luminance intensity distribution regarding each light source toward the optical axis on the plane including the pupil of the projection lens, nevertheless, since the wedge-shaped prism has a large thickness, arranging the two wedge-shaped prisms at a specified axial separation provides aggravation of compactness of the overall integrator and an increase in manufacturing cost due to uniqueness of the wedge-shaped prism.